Lignocellulose is ubiquitous in all wood species and all agricultural and forestry waste. In addition, municipal waste which typically contains about half waste paper and yard waste, is a source of lignocellulosic materials. Currently, municipal waste is buried or burned at considerable expense to the disposer or the government organization providing solid waste services.
Lignocellulosic biomass is a complex structure of cellulose fibers wrapped in a lignin and hemicellulose sheath. The ratio of the three components varies depending on the type of biomass. Typical ratios are as follows:
______________________________________ Softwoods Corn cobs R D F* ______________________________________ Cellulose 42% 40% 52% Hemicellulose 25% 36% 26% Lignin 28% 13% 20% ______________________________________ *RDF = Refuse Derived Fuel from municipal systems waste
Different woods also have different compositions. Softwoods (gymnosperms) generally have more glucommanans and less glucuronoxylans than hardwoods and grasses (angiosperms).
Cellulose is a polymer of D-glucose with .beta. 1.fwdarw.4! linkages between each of the about 500 to 10,000 glucose units. Hemicellulose is a polymer of sugars, primarily D-xylose with other pentoses and some hexoses with .beta. 1.fwdarw.4! linkages. Lignin is a complex random polyphenolic polymer. Therefore, lignocellulose represents a very cheap and readily available substrate for the preparation of sugars which may be used alone or microbially fermented to produce alcohols and other industrial chemicals.
Ethanol, one of the alcohols which can be produced from lignocellulosic biomass, has a number of industrial and fuel uses. Of particular interest is the use of ethanol as an additive to gasoline to boost octane, reduce pollution and to partially replace gasoline in the mixture. This composition is the well known commercial product called "gasohol". It has been proposed to eliminate gasoline completely from the fuel and to burn ethanol alone. Such a fuel would produce considerably less air pollution by not forming as much carbon monoxide or hydrocarbon emissions. Furthermore, gasoline is produced from crude oil which fluctuates in price, availability and is the subject of unpredictable world politics.
It has been estimated that about 1.times.10.sup.9 tons of lignocellulosic wastes are produced every year. This amount exceeds the total amount of crude oil consumed per year. In theory, if properly managed, the lignocellulose produced by the United States is sufficient to produce all of the country's needs for liquid fuel if the cellulose and hemicellulose can be completely converted into ethanol. The amount of energy theoretically obtainable from the combustion of cellulose or the glucose or alcohol derived therefrom is about 7200 BTU per pound or roughly equivalent to 0.35 pounds of gasoline. Hemicellulose has similar value when converted into sugars or ethanol. Consequently, cellulose and hemicellulose represents a readily available potential source for ethanol production.
The technology for the production of ethanol from grain and fruit for beverage purposes has been well developed for centuries. However, the costs have been relatively high compared to the cost of gasoline. Accordingly, many methods have been proposed to reduce the cost and increase the efficiency of ethanol production.
Among the techniques proposed for the production of fuel grade ethanol include the hydrolysis of cellulose to produce glucose which can be fermented to produce ethanol. Cellulose in the form of wood, newsprint and other paper, forest, agricultural, industrial and municipal wastes is quite inexpensive compared to grain, fruit, potatoes or sugarcane which is traditionally used to prepare alcohol beverages.
Cellulose hydrolysis using an acid catalyst to produce sugars has been known for decades but can be costly and requires special equipment. The sugars themselves, are somewhat labile to the harsh hydrolysis conditions and a large number of unwanted or toxic byproducts may be formed. If exposed to acid for too long, the glucose derived from cellulose degrades into hydroxymethylfurfural which can be further degraded into levulinic acid and formic acid. Xylose, which is formed from hemicellulose, is degraded by acids into furfural and then results in tars and other degradation products.
In order for acid to completely hydrolyse the cellulose and hemicellulose in a lignocellulosic substrate, degradation of the desirable sugars and formation of the toxic byproducts cannot be avoided due to kinetic constraints. To use conditions sufficiently gentle that insignificant degradation of sugars will occur does not result in complete hydrolysis of substrate. Furthermore, the acid is corrosive and requires special handling and equipment. Accordingly, in the last twenty years attention has focused on enzymatic hydrolysis of cellulose with cellulase followed by fermentation of the resulting sugars to produce ethanol which in turn is distilled to purify it sufficiently for fuel uses.
Cellulase is an enzyme complex that includes three different types of enzymes involved in the saccharification of cellulose. The cellulase enzyme complex produced by Trichoderma reesei QM 9414 contains the enzymes named endoglucanase (E.C.3.2.1.4), cellobiohydrolase (E.C.3.2.1.91) and .beta.-glucosidase (E.C.3.2.1.21). Gum et al, Biochem. Biophys. Acta, 446: 370-86 (1976). The combined synergistic actions of these three enzymes in the cellulase preparation completely hydrolyses cellulose to D-glucose.
However, cellulase can not completely degrade the cellulose found in native, unpretreated lignocellulose. It appears that the hemicellulose and lignin interfere with the access of the enzyme complex to the cellulose, probably due to their coating of the cellulose fibers. Furthermore, lignin itself can bind cellulase thereby rendering it inactive or less effective for digesting cellulose. For example, raw ground hardwood is only about 10 to 20% digestible into sugars using a cellulase preparation.
To overcome these shortcomings, applicants and others have previously disclosed a pretreatment step which attempts to degrade or remove at least a portion of the hemicellulose and/or lignin. The result of the pretreatment has been greater digestibility of the cellulose by a cellulase complex. One such pretreatment has been the use of a comminution step and a combination of heat and acid for a period of time which hydrolyses most of the hemicellulose, thus rendering the cellulose digestible by a cellulase complex.
In the prior art, by using such a pretreatment step, little lignin has been removed. See Grohmann et al, Biotechnology and Bioengineering, Symp. No. 17: 135-151 (1986), Torget et al, Applied Biochemistry and Biotechnology, 24/25: 115-126 (1990), Torget et al, Applied Biochemistry and Biotechnology, 28/29: 75-86 (1991) and Torget et al, Applied Biochemistry and Biotechnology, 34/35: 115-123 (1992).
Lignin removal from cellulosic fibers by a caustic (alkali) is the basis for Kraft pulping and paper making. However, such techniques are aimed at conserving the polymeric carbohydrate integrity and thus do not produce simple sugars, and do not separate the hemicellulose from the cellulose.
The difficulty of degrading the hemicellulose and cellulose remains. Conditions optimized to remove hemicellulose are not very effective at removing lignin and vice versa. Therefore, the cellulose remaining after prehydrolysis may be less than ideally separated from the other constituents and may retard digestion by cellulase. Furthermore, the costs involved for the pretreatment step can be significant. The chemicals used to alter pH and the steam required to heat the lignocellulose add a cost to the process. The greater the duration of the prehydrolysis step, the greater cost in heat to maintain the temperature; and the slower the overall process, also the greater cost in time and equipment.
Elian et al, U.S. Pat. No. 2,734,836, discloses a process where acid is used to pretreat cellulosic materials to extract pentoses using acetic acid. The material is sprinkled with the acid and heated to 80.degree.-120.degree. C. and the acid is recycled through the cooker in a manner to preserve the cellulose fibers. The residual material is used in conventional pulping.
Richter, U.S. Pat. No. 3,532,594, discloses digesting cellulosic material by soaking the solids in an alkaline liquid, and then applying steam in a gas phase to heat the material. The material is cooled and washed to recover cellulosic fibers. The digestion of the non-cellulose occurs in the gas phase as wood chips descend in the reactor. No reaction is occurring in the countercurrent washing step.
Eickemeyer, U.S. Pat. No. 3,787,241, discloses a percolator vessel for decomposing portions of wood. The first stage is the hydrolysis of hemicellulose to xylose using 1% sulfuric acid (column 4, lines 23-34) and then acid hydrolysis of cellulose occurs later. Lignin remains in the reactor throughout the hydrolysis and is removed at the end.
Pfeiffer, U.S. Pat. No. 4,226,638, discloses an acid pretreatment of young plants where the acid is not permitted to saturate the young plants. Column 3, line 59 specifically states that the amount of saturation is less than 70% and line 67 states that 60% of the xylan is hydrolysed. The wash step is performed by water. The goal is to extract xylose form plant material.
Brink, U.S. Pat. No. 4,384,897 and later U.S. Pat. No. 5,221,357, uses a nitric acid two stage hydrolysis where the first stage is under mild conditions to hydrolyse the hemicellulose. (pH 2-3, 140.degree.-220.degree. C.) Later harsher acidic conditions are used to hydrolyse the cellulose present which is then washed free from the lignin. No lignin is removed during the hydrolysis. The liquid and biomass particles travel in a cocurrent flow pattern according to column 10 lines 1-5 of the more recent patent.
Elmore, U.S. Pat. No. 4,436,586, describes a method for treating wood chips by acid prehydrolysis to extract xylose for fermentation to ethanol and preserves the cellulose fibers and lignin for kraft pulping of the residual solids. The pretreatment is in a reactor with filtering screens and recycling of the acid.
Tourier et al, U.S. Pat. No. 4,511,433, describes a continuous extraction process where wood is placed in a reactor 1 and is retained by filter 2 where a strongly acidic solution is passed through the system to remove pentoses. The acidic solution is mixed with a phenol compound in a different phase to remove lignin. The two phase system is needed for Tourier to achieve their results.
Neves, U.S. Pat. No. 4,564,595, discloses a process which includes cellulose cooking and degradation with acid. The process separates hemicellulose from the pretreated material and in a separate step removes lignin. According to the second paragraph in column 7, agitation of the mixture appears essential to adequate extraction.
Sherman et al, U.S. Pat. No. 4,612,286 and later U.S. Pat. No. 4,668,340, hydrolyse the hemicellulose in a high solids system by a complex process of passing a 20-40% solids slurry of biomass into the bottom of a reactor and passing acid through a series of screens and solid-liquid separators to wash the hydrolysed C5 sugars in a separate location which are then withdrawn as hydrolyzate. According to column 2, line 37, the acid concentration in the reaction chamber is about 2% to 10%. Conditions are chosen so that cellulose is not hydrolysed. The remaining solids include cellulose and lignin which are burned or if separated, must be done in a separate step.
Wright, U.S. Pat. No. 4,615,742, discloses a series of hydrolysis reactors. Some of these are prehydrolysis reactors and are for removing hemicellulose while others are for hydrolysis. Because the contents move in a series, the duration of each step is the same. The process does not remove lignin from the solids and multiple reactors are required. Within each reactor is a predefined set of conditions.
Rehberg, U.S. Pat. No. 4,995,943, pretreats biomass with high pressure carbon dioxide and then suddenly releases it to open up the material to allow better degradation of cellulose. The gas applied is described as anhydrous and therefore should not contain carbonic acid other than that which forms in situ. The purpose of the Rehberg process is unrelated to any prehydrolysis process as water is not present in appreciable amounts.
Grohmann et al, U.S. Pat. No. 5,125,977, demonstrated that different xylans could be removed during prehydrolysis under two different conditions by prehydrolyzing the substrate, centrifuging the mixture to recover xylose from certain xylans in the supernatant. The solids were remixed with additional acid, the prehydrolysis completed and a second xylose solution produced by apparently different xylans in the hemicellulose. The two types of xylan are hydrolysed by two different conditions which are optimized for each xylan.